1. Field of the Invention
The present invention relates to a display device and a driving method for the display device, and particularly relates to a control circuit of a display panel using a light emitting element in a pixel. A control circuit of a memory is a circuit that controls writing to and reading from the memory, typified by an SRAM (Static Random Access Memory).
Note that the control circuit of a display panel mentioned here is a circuit which converts received video data so that gray scale expression in a pixel of the display panel becomes possible, and writes in a storage means and outputs the video data read from the storage means to the display panel for displaying.
Note that the display device is structured by a display and a peripheral circuit that inputs signals to the display.
2. Description of the Related Art
In recent years, as a display device for replacing liquid crystal display devices (LCD), there is a light emitting device that is structured by a display panel in which a light emitting element is placed in every pixel, and a peripheral circuit that inputs signals to the panel, and that carries out image display by controlling light emission of the light emitting elements.
The development of a light emitting device using a module structured by light emitting elements arranged in a matrix form is widely pursued, and EL elements are receiving attention.
In such a light emitting device, two or three TFTs (thin film transistor) are typically placed in every pixel. By controlling on/off of those TFTs, luminance and light emission/non-emission of a light emitting element of each pixel are controlled. Further, a driver circuit for controlling on/off of the TFTs of each pixel is provided in a peripheral portion of a pixel portion of the display panel.
Here, a variety of elements can be used for a light emitting element of the present specification. For example, an OLED element; an inorganic light emitting diode element or another light emitting diode element; an inorganic EL (Electroluminescence) element or another solid system light emitting element; an FED element or another vacuum system light emitting element, or the like are given. Note that an OLED element includes an anode, a cathode, and an organic light emitting layer interposed between the anode and the cathode.
As a method for expressing gray scale of a pixel of a structure such as that of the foregoing, there are two main methods of an analog method and a digital method. The digital method is advantageous compared to the analog method in that it is resistant to variation in TFT characteristics. As a gray scale expression method of the digital method, a time gray scale method and an area gray scale method are given.
The time gray scale method is a method for expressing gray scale by controlling a period in which each pixel of a display device emits light. If a period in which one image is displayed is one frame period, the one frame period is divided into a plurality of sub-frame periods. Gray scale of each pixel is expressed by having each pixel be lighted or not lighted in each sub-frame period as well as changing a display period of each sub-frame period, and controlling a total period of light emission by selecting a combination of sub-frame periods in which each pixel lighted.
The area gray scale method is a method for expressing gray scale by controlling an area of a portion in each pixel of the display device that emits light. Specifically, the area gray scale method is a method for expressing gray scale of each pixel by dividing each pixel into sub-pixels and changing the number of sub-pixels that emit light.
Note that for a display device that expresses gray scale by the time gray scale method or the area gray scale method such as the foregoing, a control circuit that carries out format conversion of received video data into video data for time gray scale display or video data for area gray scale display, and outputs to the display panel is needed.
As the control circuit of such a display device, there is a circuit for a display device of a time gray scale method mentioned in Patent Document 1: Japanese Published Patent Application No. 2004-163919 for example, which is shown in FIG. 11. The control circuit in FIG. 11 is structured by a format conversion circuit including a format conversion portion 1401 that converts a first video data into a second video data for time gray scale; a first video memory 1402 and a second video memory 1403 for storing the format-converted second video data; a display control circuit including a display control portion 1404 that reads data from the first video memory 1402 or the second video memory 1403 and transmits the data to a display panel 1406; and a selection circuit 1405 for selecting a memory to which data is written and a memory from which data is read.
FIG. 12 shows a timing chart of a conventional control circuit. Video data input to the format conversion portion 1401 is converted to data suitable for the time gray scale method, and using the selection circuit 1405, data writing and data reading are alternated every one frame period. In other words, using the first video memory 1402 and the second video memory 1403, at a certain point in time, one memory is used for reading the video data and the other is used for writing.
At the same time as reading the first video data stored in the first video memory 1402 to a display control portion, a second video data corresponding to a subsequent frame period is written to the second video memory 1403 via the selection circuit.
In this manner, the control circuit of the display device in FIG. 11 includes the first video memory 1402 and the second video memory 1403 each of which can store digital video data for one frame period, and the second video data is sampled by alternately using the first video memory 1402 and the second video memory 1403.